Fuel 153 (2015) 361–369 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Promotional effect of CeO2 modiﬁed support on V2O5–WO3/TiO2 catalyst for elemental mercury oxidation in simulated coal-ﬁred ﬂue gas Lingkui Zhao, Caiting Li ⇑, Jie Zhang, Xunan Zhang, Fuman Zhan, Jinfeng Ma, Yin’e Xie, Guangming Zeng College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China h i g h l i g h t s g r a p h i c a l a b s t r a c t Nano-sized TiO2–CeO2 mixed oxides Mechanism of CeO2 modiﬁed V2O5–WO3/TiO2 catalysts for elemental mercury oxidation. is used as support material for Hg0 oxidation. 0 The Hg oxidation activity of VWTi was promoted by support modiﬁcation. There was synergistic effect between V2O5 and CeO2 on Hg0 oxidation. 0 Hg oxidation over V0.80WTiCe0.25 follows a Mars–Maessen mechanism. a r t i c l e i n f o Article history: Received 8 October 2014 Received in revised form 21 January 2015 Accepted 2 March 2015 Available online 20 March 2015 Keywords: V2O5–WO3/TiO2 Catalytic oxidation Elemental mercury Flue gas CeO2 a b s t r a c t In order to enhance the catalytic activity for elemental mercury (Hg0) oxidation without the aid of HCl, CeO2 was added into the support to modify V2O5–WO3/TiO2 catalysts. The performance of V2O5–WO3/ TiO2–CeO2 (VWTiCe) catalysts on Hg0 oxidation as well as the catalytic mechanism was also studied. The catalysts were characterized by BET, XRD and XPS techniques. The results showed that the performance on Hg0 oxidation was promoted by the introduction of CeO2. NO and SO2 has a promoting effect on Hg0 oxidation in the presence of O2. Besides, the inhibitive effect of NH3 on Hg0 oxidation was conﬁrmed by NH3 consuming the surface oxygen of catalyst. The addition of CeO2 improved the ability to resist H2O. Results also indicated that the Hg0 oxidation efﬁciencies of V0.80WTiCe0.25 catalysts were thought to be aided by synergistic effect between V2O5 and CeO2. Hg0 oxidation over V0.80WTiCe0.25 follows a Mars–Maessen mechanism where lattice oxygen of V2O5 reacts with adjacently absorbed Hg0. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Mercury pollution has received considerable attention from environmental researchers due to its high volatility, long persistence, and strong bioaccumulative properties [1–3]. Coal ⇑ Corresponding author at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. Tel./fax: +86 731 88649216. E-mail addresses: [email protected], [email protected] (C. Li). http://dx.doi.org/10.1016/j.fuel.2015.03.001 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved. combustion is a signiﬁcant fraction of anthropogenic source of mercury emission . In January 2013, 140 nations adopted the ﬁrst legally binding international treaty to set enforceable limits on emissions of mercury and exclude, phase out, or restrict some products that contain mercury . A number of technologies have been used for mercury control from coal combustion ﬂue gas, such as sorbent injection , catalytic oxidation  and photochemical oxidation . It was reported  that the efﬁciency of mercury control methods 362 L. Zhao et al. / Fuel 153 (2015) 361–369 depends largely on the form of mercury. Mercury in the ﬂue gas from coal combustion is often classiﬁed into three forms, i.e. elemental mercury (Hg0), oxidized mercury (Hg2+), and particleassociated mercury (HgP) [10–12]. Hg2+ and HgP can be easily removed by existing air pollution control installations such as fabric ﬁlters (FF), cold-side or hot-side electrostatic precipitators (ESPs), wet or dry ﬂue gas desulfurization (FGD) and selective catalytic reduction (SCR) . However, Hg0 is much difﬁcult to be capture by air pollution control devices because of its high volatility and nearly insolubility in water [14–16]. Therefore, development of viable technologies for the effective conversion of Hg0 to Hg2+ has become the focus of many investigations in recent years. With extensive application of the SCR technology in coal-ﬁred power plants, lots of full-scale tests have been carried out to evaluate the performance of the SCR catalysts on Hg0 oxidation [16–19]. As a typical commercial NH3–SCR catalyst, V2O5–WO3/TiO2 catalysts not only show high catalytic activity for NO reduction but also exhibit the co-beneﬁt of promoting mercury oxidation . Consequently, a low cost option for control of mercury from coal ﬁred power plants can be achieved by this co-beneﬁt of SCR installation. It is well-known that the involved oxidants are mainly oxygen and chlorine in real ﬂue gas. However, in China, the chlorine content of feed-coal (63–318 mg/kg) is much lower than that of US coals (628 mg/kg) [21–23]. Thus, catalytic oxidation of Hg0 using gaseous oxygen as the oxidant is an environment-friendly and economical method for Hg0 control. However, the conventional SCR catalysts were not effective enough for Hg0 oxidation in the absence of HCl. Accordingly, how to improve the catalytic activity for Hg0 oxidation without HCl has become an important research scope the co-beneﬁt performance of SCR catalysts. Up to now, many kinds of catalysts have been investigated for SCR such as metal oxides supported on TiO2 [24–26]. However, TiO2 still exhibits some drawbacks where improvements can be made, such as low surface area [27,28]. In general, appropriate active sites, high-surface-area supports, and their interaction are important to the activity of the catalysts . Thus, in order to improve the properties of TiO2, the sol–gel method was used to synthesize nano-sized CeO2–TiO2 carriers. It has been reported that TiO2– CeO2 mixed oxide is a promising catalyst for the SCR of NO with NH3 in the presence of oxygen [30,31]. A Ce–Ti based (CeO2– TiO2) catalyst showed excellent NH3–SCR activity, high N2 selectivity, broad operation temperature window, and high resistance to space velocity . What is more, for CeTi catalyst, the combination of SO2 and NO without HCl resulted in high Hg0 oxidation efﬁciency . Accordingly, CeO2–TiO2 mixed oxide is extensively studied for emission control. It is well known that cerium has two stable oxidation states (Ce3+ and Ce4+), which could provide cerium with signiﬁcant oxygen storage capability through the redox shift between the two oxidation states . Meanwhile, CeO2 has been studied for Hg0 oxidation and it could enhance catalytic activity due to its oxygen storage capability by storing or releasing O via the Ce4+/Ce3+ redox couple . Furthermore, CeO2 based catalysts were reported to have resistance to the effect of water vapor [30,33]. Consequently, CeO2 is widely used in composite materials and as a catalyst support material . However, catalysts with nano-sized TiO2–CeO2 composite oxides carrier such as V2O5– WO3/TiO2–CeO2 for Hg0 oxidation without HCl and the catalytic mechanism have rarely been reported. The present work aimed to improve mercury oxidation activity without the aid of HCl as well as reveal the catalytic mechanism of CeO2 modiﬁed support on V2O5–WO3/TiO2 catalysts. A series of experiments and characterizations were carried out to probe into the superiority of catalyst with nano-sized TiO2–CeO2 composite oxide carriers. Besides, the effects of individual ﬂue gas components on Hg0 oxidation were also studied. 2. Experimental section 2.1. Reagents All the reagents used in this work were analytical pure grade (AR), including: The anatase-type nanosize TiO2 powder (99.8%, Chengdu Ai Keda Chemical Technology Co.), cerium nitrate (99.0%, Kemiou Chemical Reagent Co.), anhydrous ethanol (99.7%, Kemiou Chemical Reagent Co.), nitric acid (68.0%, Sinopharm Chemical Reagent Co), butyl titanate (99.0%, Kemiou Chemical Reagent Co.), ammonium wolframate (90.0%, Sinopharm Chemical Reagent Co), ammonium metavanadate (99.0%, Kemiou Chemical Reagent Co.) and Oxalic acid (99.5%, Sinopharm Chemical Reagent Co). Ultrapure water was applied to prepare the required solutions. 2.2. Preparation of catalysts TiO2–CeO2 composite oxide carriers were prepared by a sol–gel method. Speciﬁcally as follows: a certain amount of cerium nitrate, anhydrous ethanol (0.6 mol), ultrapure water (1.9 mol) and nitric acid (0.1 mol) were mixed in the beakers, which were stirred for 30 min and labeled as A solution; a requisite amount of butyl titanate (0.1 mol) were stirred in anhydrous ethanol (2.3 mol) for 30 min and labeled as B solution; then B solution was added drop-wisely to A solution with vigorous stirring. After stirring for 5 h at room temperature, the sol was concentrated in 40 °C water bath and subsequently dried at 80 °C for 24 h. The gel was calcinated at 500 °C for 5 h in air. TiO2–CeO2 composite oxide carriers was denoted as TiCea (‘‘a’’ represents the CeO2/TiO2 mass ratio; a = 0.11, 0.25, 0.43, 0.67, 1.00). Stoichiometric amount of ammonium wolframate solution and ammonium metavanadate were mixed in the oxalic solution of desired proportions, which was labeled as C solution. V2O5–WO3/ TiO2–CeO2 catalysts were prepared by the dispersal of a certain mass of TiCea powder into 50 ml C solution at 80 °C to obtain the slurry. The slurry was stirred for 2 h, after that, the mixture was exposed to an ultrasonic bath for 2 h, dried at 105 °C for 12 h and subsequently calcined at 500 °C for 3 h in air. Finally, CeO2 doped catalyst V2O5–WO3/TiO2 was obtained and abbreviated as VxWTiCea (‘‘x’’ represents the V2O5/(V2O5 + WO3 + TiO2 + CeO2) mass ratio, x = 0.40 102, 0.60 102, 0.80 102, 1.00 102, 1.20 102). Meanwhile, WO3/TiO2 (WTi) catalysts, V2O5–WO3/ TiO2 (VWTi) catalysts and WO3/TiO2–CeO2 (WTiCe) catalysts were synthesized by impregnation method, and the processes were the same as the foregoing conditions. The mass loading of WO3 on all catalysts was 8%. 2.3. Catalytic activity measurement The experimental setup for evaluating Hg0 oxidation on the samples was shown in Fig. 1. The catalytic activities of Hg0 oxidation were tested at 80–350 °C in a ﬁxed bed reactor under atmospheric pressure. The reaction temperature was controlled by a digital temperature controller. 0.5 g catalyst samples were packed in the center of the quartz tube (i. d. 20 mm). The simulated ﬂue gas (SFG) was consisted of 70.00 lg/m3 Hg0, 500 ppm SO2 (20.4% SO2 + 79.6% N2), 1000 ppm NO (20.0% NO + 80.0%N2), 12%CO2 (99.999%), 5% O2 (99.999%) and balanced gas N2 (99.999%). The feed gas was controlled by mass ﬂow meters and injected into the reactor at a total rate of 1 L min1 with a gas hourly space velocity of 1.0 105 h1. A constant quantity of Hg0 vapor was supplied into the gas steam, with an Hg0 permeation tube (VICI Metronics, USA) which was immersed in a water bath. A peristaltic pump transferred water into the Teﬂon tube wrapped with a L. Zhao et al. / Fuel 153 (2015) 361–369 temperature-controlled heating tape and then water vapor was generated. 100 ml pure N2 took the water vapor along and mixed with the ﬂue gas. Besides, In order to avoid adsorption of Hg0 and condensation of water vapor on the inner surface, all Teﬂon lines that Hg0 and H2O (g) passed through were heated up to 120 °C. The Hg0 concentration in the inlet and outlet gas was online measured by a RA-915 M Mercury Analyzer (LUMEX Ltd, Russia) which can measure solely the concentration of Hg0. Meanwhile, Hg0 concentration was recorded after the process had reached equilibrium. The time was more than 2 h. The experiment was carried out to ensure the reliability of data obtained from the test on catalyst performance. Result showed that VWTiCe catalyst possessed excellent stability on Hg0 oxidation and could perform well in presupposed time quantum of 2 h for experiment. Water vapor was removed by the condenser before proceeding to the mercury analyzer. Before the test, the ﬂue gas bypassed the reactor and Hg0 concentration in the inlet ([Hg0]in) was measured. Then, the gas ﬂow was switched to pass through the catalysts and Hg0 concentration in the outlet ([Hg0]out) was measured. The loss of Hg0 concentration over the catalysts should be due to the oxidation or adsorption of Hg0. In order to avoid possible bias because of Hg0 adsorption, at the beginning of the Hg0 catalytic oxidation tests, the catalysts were ﬁrst saturated with the established [Hg0]in under N2 atmosphere at room temperature [23,33,37]. The adsorption test result indicated that the capacities of catalyst samples to adsorb Hg0 were negligible at room temperature. Therefore, Hg0 oxidation efﬁciency (Eoxi) over the catalysts was quantiﬁed by the following equation: Eoxi ð%Þ ¼ ½Hg0 in ½Hg0 out ½Hg0 in 100% ð1Þ 2.4. Characterization Brunauer–Emmett–Teller (BET) surface area, average pore size, and average pore volume of the samples were analyzed by Micromeritics Tristar II 3020 analyzer (Micromeritics Instrument 363 Crop, USA). Each sample was degassed in vacuum at 180 °C for 5 h. The speciﬁc surface areas were calculated by the BET method. The average pore diameter and average pore volume were obtained from the desorption branches of N2 adsorption isotherm and calculated by the BJH (Barrett–Joyner–Halenda) formula. X-ray diffractogram (XRD) measurements were carried out on Rigaku rotaﬂex D/Max-C powder diffractometer (Rigaku, Japan) to examine the crystallinity and dispersivity of each species on the support. The XRD patterns were used nickel-ﬁltered Cu Ka radiation (k = 0.1543 nm) in the range of 10–80° (2h) with a step size of 0.02°. X-ray Photoelectron Spectroscopy (XPS) analysis was carried out at room temperature on a K-Alpha 1063 X-ray photoelectron spectrometer (Thermo Fisher Scientiﬁc, UK) with an Al Ka X-ray source. The observed spectra were calibrated with the C 1s binding energy (BE) value of 284.6 eV. 3. Results and discussion 3.1. Catalytic activity tests The comparison of catalytic performance of different catalysts as a function of temperature from 80 to 350 °C was shown in Fig. 2. Eoxi over WTi was below 40% in the entire temperature range. On the TiCe0.25 catalyst, Eoxi increased with temperature from 80 to 250 °C, and then decreased when temperature further increased from 250 to 350 °C. V0.80WTiCe0.25 performed the best mercury oxidation and approximately 88.93% mercury oxidation efﬁciency was obtained at 250 °C. Additionally, it was clearly found that the addition of ceria noticeably expanded the active temperature window and also improved the catalytic performance on Hg0 oxidation. In the whole temperature range, Eoxi over V0.80WTiCe0.25 was higher than that over WTiCe0.25, V0.80WTi and TiCe0.25 catalyst. This result demonstrated that the combination of CeO2 and V2O5 resulted in signiﬁcant synergy for Hg0 oxidation. The effect of CeO2 modiﬁed on the performance of V0.80WTiCea was displayed in Fig. 3. The addition of CeO2 signiﬁcantly enhanced Fig. 1. Schematic diagram of the experimental setup. 364 L. Zhao et al. / Fuel 153 (2015) 361–369 the mercury oxidation activity of V0.80WTiCea. For instance, Eoxi of V0.80WTi was only 42.59%, while the minimal Eoxi still had 65.10% over V0.80WTiCea. Moreover, Catalyst with a CeO2/TiO2 mass ratio of 0.25 performed the best activity and 88.93% mercury oxidation efﬁciency was obtained. However, further increase of CeO2/TiO2 mass ratio, Hg0 oxidation efﬁciency would be weakened. Accordingly, the optimal CeO2/TiO2 mass ratio was about 0.25. To investigate the synergetic interaction between V2O5 and CeO2, the effect of various V2O5 loading from on the performance of VxWTiCe0.25 was also studied, and the results were depicted in Fig. 4. For all VxWTiCe0.25, Eoxi increased with temperature from 80 to 250 °C and then decreased when temperature further increased. In the temperature range (250–350 °C), increase of V2O5 loading yielded more Hg0 oxidation. This illustrated that the V2O5-rich catalyst showed superior activity. However, activity difference between V0.80WTiCe0.25 and V1.00WTiCe0.25 was relatively small in comparison with that of other catalysts at 250 °C. Besides, V1.00WTiCe0.25 showed almost the same activity as V1.20WTiCe0.25 at 200 °C. It is worth noting that V0.80WTiCe0.25 displayed excellent performance for Hg0 oxidation with temperature from 80 to 150 °C. According to the literature , CeO2 was active for Hg0 oxidation at low temperature. Above results demonstrated that Hg0 oxidation activities of VxWTiCe0.25 were aided by synergistic effect between V2O5 and CeO2. Fig. 3. Effect of CeO2 modiﬁed on the performance of V0.80WTiCea. Reaction conditions: 1000 ppm NO, 500 ppm SO2, 5% O2, 12% CO2, N2 as balance gas. 3.2. Characterization of V0.80WTiCe0.25 3.2.1. Analysis of speciﬁc area (BET) and XRD patterns The BET surface area, BJH pore volume and average pore volume of different samples were summarized in Table 1. From the table, WTiCe0.25 and V0.80WTiCe0.25 exhibited higher speciﬁc surface areas. The introduction of CeO2 increased the surface area and pore volume but lowered the average pore diameter (Fig. 5a). The results suggested the addition of CeO2 was beneﬁcial to the speciﬁc area accretion. The high speciﬁc surface area of the TiO2–CeO2 composite oxides was due to the amorphous structure that the oxides formed through the sol–gel procedure . Furthermore, as the impregnation of V2O5, the speciﬁc surface area and pore volume of V0.80WTiCe0.25 decreased. It might be caused by deposited active oxides, which penetrated into the pores of the support. In addition, as displayed in Fig. 5b and c, for V0.80WTiCe0.25, there was signiﬁcant hysteresis between adsorption and desorption isotherms, which is usually an indication of mesoporous materials. This clariﬁed that the mesoporous structure was formed by Fig. 4. Effect of VxWTiCe0.25 catalysts of various V loadings. Reaction conditions: 1000 ppm NO, 500 ppm SO2, 5% O2, 12% CO2, N2 as balance gas. aggregation of nano-particles. This structure might facilitate mass transfer in the catalytic reaction . The XRD patterns of the catalysts were displayed in Fig. 6. The characteristic peaks of V2O5 and WO3 could be hardly detected for all catalysts, which were due to the widely dispersion and poorer crystalline on the surface. From the XRD ﬁgure, it can be seen that the diffraction peaks of WTi and V0.80WTi showed typical anatasephase TiO2. The diffraction line of WTi and V0.80WTi were narrow and sharp, which indicated the formation of large TiO2 crystal particles . In the pattern of the WTiCe0.25 and V0.80WTiCe0.25, only the broadened diffraction peak of anatase-phase TiO2 was observed. It also can be seen that the height of half-peak breadth of TiO2 on Table 1 The surface area, pore volume and pore diameter of the samples. Fig. 2. Comparison of catalytic performance of different catalysts. Reaction conditions: 1000 ppm NO, 500 ppm SO2, 5% O2, 12% CO2, N2 as balance gas. Catalysts BET surface area (m2/g) Pore volume (cm3/g) Average pore diameter (nm) WTi V0.80WTi WTiCe0.25 V0.80WTiCe0.25 38.4855 41.6403 122.9062 116.6715 0.1322 0.1432 0.2931 0.2809 13.7379 13.7511 9.5404 9.6293 L. Zhao et al. / Fuel 153 (2015) 361–369 365 Fig. 5. Physical properties of different catalysts. (a) Particle size distribution of WTi, V0.80WTi, WTiCe0.25 and V0.80WTiCe0.25; (b) N2 adsorption and desorption isotherms of WTiCe0.25; (c) N2 adsorption and desorption isotherms of V0.80WTiCe0.25. WTiCe0.25 and V0.80WTiCe0.25 were much lower than that on WTi and V0.80WTi. This revealed that the crystal particle of TiO2 on WTiCe0.25 and V0.80WTiCe0.25 were much smaller than that on WTi and V0.80WTi. Moreover, cubic CeO2 was not observed in WTiCe0.25 and V0.80WTiCe0.25. This demonstrated that CeO2 probably only contained amorphous phase or crystallite phase with very small particle size . Thus, according to the BET and XRD results, the addition of an appropriate amount of CeO2 was beneﬁcial not only for the formation of the TiO2 crystal phase but also for the dispersion of active sites over the carrier. Besides, high concentration of amorphous or highly dispersed crystalline CeO2 should be a rational reason for the excellent performance of the V0.80WTiCe0.25. 3.2.2. Element valences of V0.80WTiCe0.25 To determine the oxidation states of the element in these materials and to get a better understanding nature of the Fig. 6. XRD patterns of WTi, V0.80WTi, WTiCe0.25 and V0.80WTiCe0.25. interactions in the catalyst system, the catalysts were investigated by XPS technique. The XPS spectra for O 1s for fresh WTi, WTiCe0.25, V0.80WTi and V0.80WTiCe0.25 were shown in Fig. 7. It could be seen that the introduction of CeO2 increased the binging energy of O. The peak appeared at low binding energy (529.5– 530.0 eV) could be assigned to be the lattice oxygen (denoted as Ob) , while the binding energy of 531.0–531.6 eV was assigned to the chemisorbed oxygen (denoted as Oa), such as O2 and O belonging to defect oxide or hydroxyl like group . Oa was often thought to be the most active oxygen and played an important role in oxidation reaction [41,42]. In this study Oa ratio of WTiCe0.25 (42.63%), calculated by Oa/(Oa + Ob), was higher than that of WTi (33.21%), which meant that CeO2 was helpful for mercury oxidation. CeO2 was easy to form labile oxygen vacancies and particularly the relatively high mobility of bulk oxygen species, which may conduce to the improvement of chemisorbed oxygen . Moreover, Oa ratio of V0.80WTiCe0.25 (48.32%) was higher than that of WTiCe0.25 (42.63%) and V0.80WTi (41.19%). This result meant that there was synergistic effect between V2O5 and CeO2, which resulted in more surface oxygen vacancies. It also meant that V0.80WTiCe0.25 might have better activity for mercury oxidation than WTi, WTiCe0.25 and V0.80WTi. Thus, the addition of CeO2 has a positive effect on mercury oxidation reaction. This conclusion was in good agreement with the results of the activity tests for these catalysts. The Ce 3d spectra of fresh V0.80WTiCe0.25 and spent V0.80WTiCe0.25 investigated in this study were presented in Fig. 8. The bands labeled u1 and v1 represent the 3d104f1 initial electronic state, corresponding to Ce3+, whereas the peaks labeled u, u2, u3, v, v2, and v3 represent the 3d104f0 state of Ce4+ ions . It could be clearly seen the high ratio of Ce3+ (40.27%) over fresh V0.80WTiCe0.25. In comparison with the fresh sample, the ratio of Ce3+ increased to 43.82% on spent V0.80WTiCe0.25. This implied that the conversion from Ce4+ to Ce3+ was predominant within the redox shift between Ce3+ and Ce4+. Within the redox shift, labile oxygen vacancies and bulk oxygen species with relatively high 366 L. Zhao et al. / Fuel 153 (2015) 361–369 strong interactions between V2O4 and CeO2. It was most likely that V4+ could be oxidized to V5+, which was beneﬁted from the transformation from Ce4+ to Ce3+ ions. The Hg 4f spectra of spent V0.80WTiCe0.25 and spent V0.80WTiCe0.25 were presented in Fig. 10. The Hg 4f spectrum of spent V0.80WTiCe0.25 exhibited contributions from two components. According to the reported binding energies for Hg 4f on reference , the components can be ascribed to Hg0 and HgO with two peaks locating at 102.6 eV and 108.5 eV, respectively. Therefore, the XPS peak intensities and binding energies of Hg 4f line indicated that mercury was present as Hg0 and HgO on the spent V0.80WTiCe0.25. It was concluded that Hg0 was ﬁrstly adsorbed onto the active sites of the catalyst, and subsequently oxidized to HgO. It was very likely that Hg0 oxidation of V0.80WTiCe0.25 could be mainly inﬂuenced by the O species on the samples surface. However, the absence of peaks for Hg adsorbed over the V0.80WTi indicated that the concentration of Hg adsorbed was below the capabilities of XPS equipment detection or the adsorbed Hg desorbed from the surface of catalyst. This results conﬁrmed that CeO2 modiﬁed V0.80WTi might have better activity for the adsorption and oxidation of Hg0 than unmodiﬁed V0.80WTi. In other words, the addition of CeO2 not only improved the oxygen storage, but also enhanced the redox activity through the interaction between V2O5 and CeO2. 3.3. Effect of ﬂue gas constituents on Hg0 oxidation Fig. 7. O1s XPS spectra for WTi, V0.80WTi, WTiCe0.25 and V0.80WTiCe0.25. To explore the roles of individual ﬂue gas components in Hg0 oxidation, the effects of reactant gas composition on activity were studied. The experimental conditions are listed in Table 2. Experiments were conducted at 250 °C, by individual ﬂue gas components balanced in pure N2 and/or in combination with O2. The results were shown in Fig. 11. mobility could be easily generated . Furthermore, it was reported that the lattice oxygen defects over catalyst surface would be improved by increasing the content of Ce3+, which improved the redox transformation between Ce3+ and Ce4+ . Meanwhile, two peaks at binding energy of 517.2 eV and 515.9 eV for the fresh V0.80WTiCe0.25 (Fig. 9) represented V2O4 species with V4+ and V2O5 species with V5+, respectively [46,47]. However, only V5+ was detected on spent V0.80WTiCe0.25. It was assumed that V4+ could be oxidized to V5+ over the sample. This phenomenon implied V2O4 and CeO2 were in a partially reduced state on the surface of catalyst, which might be attributable to the presence of 3.3.1. Effect of O2 O2 promoted Hg0 oxidation. In pure N2 gas ﬂow, Eoxi was low which should be due to gas-phase or weakly adsorbed Hg0 reacting with lattice oxygen to form mercuric oxide . Nevertheless, Eoxi increased from 41.98% to 71.13% when 5% O2 was added to the gas ﬂow. Obvious increase of Eoxi was detected when O2 concentration further increased to 10% as well. It has been reported that lattice oxygen of the metal oxides can serve as the oxidant of Hg, forming mercuric oxide (HgO) . Eoxi was low may be attributed to the consumption of the lattice oxygen. O2 (g) can replenish the consumed chemisorbed oxygen and regenerate the lattice oxygen, Fig. 8. Ce3d XPS spectra for fresh – V0.80WTiCe0.25 and spent – V0.80WTiCe0.25 (the sample was subjected to Hg0 oxidation under simulated ﬂue gas with the temperature of 250 °C). Fig. 9. V2P XPS spectra for fresh – V0.80WTiCe0.25 and spent – V0.80WTiCe0.25 (the sample was subjected to Hg0 oxidation under simulated ﬂue gas with the temperature of 250 °C). L. Zhao et al. / Fuel 153 (2015) 361–369 367 Fig. 10. Hg4f XPS spectra for spent – V0.80WTiCe0.25 and spent – V0.80WTiCe0.25 (the sample was subjected to Hg0 oxidation under simulated ﬂue gas with the temperature of 250 °C). which serves as the Hg0 oxidant . Hence, obvious increase of Hg0 oxidation efﬁciency was detected when 5% O2 (g) was introduced to the pure N2 carrier gas or even when O2 (g) concentration further increased to 10%. 3.3.2. Effect of NO Addition of 500 ppm NO has an enhancing effect on Hg0 oxidation under pure N2 atmosphere. Compared to the results obtained under N2 atmosphere, the oxidation Hg0 concentration was higher when NO was present in the reactor. This may be due to the addition of CeO2 in the catalyst which can adsorb and oxidation NO. Jin et al.  found that a fraction of NO reacted with the surface oxygen to form NOx species, which could enhance Hg0 oxidation . However, the addition of 1000 ppm NO into pure N2 resulted in a slight decrease of Eoxi from 75.10% to 68.42%. Nonetheless, adding 5% O2 and 1000 ppm NO to the ﬂow gas improved the catalytic performance. It was hypothesized that lattice oxygen might participate in NO oxidation. The reaction consumed surface oxygen which resulted in a slight decrease of Hg0 oxidation at 1000 ppm NO. Hg0 oxidation would be increased once surface oxygen was enough for NO and Hg0 oxidation. Therefore, NO has a promotional effect on Hg0 oxidation in the presence of O2. 3.3.3. Effect of SO2 The addition of 500 ppm SO2 slightly promoted the Hg0 oxidation efﬁciency. 500 ppm SO2 added to gas stream with O2 (g) also Table 2 Experimental conditions. Experiments Catalysts Flue gas components (1 L min1) Set I V0.80WTiCe0.25 N2 N2 + 5% O2 N2 + 10% O2 N2 + 500 ppm NO N2 + 1000 ppm NO N2 + 1000 ppm NO + 5% O2 N2 + 500 ppm SO2 N2 + 1000 ppm SO2 N2 + 500 ppm SO2 + 5% O2 SFG:N2 + 1000 ppm NO + 500 ppm SO2 + 12% CO2 + 5% O2 SFG + 8% H2O N2 + 1000 ppm NH3 N2 + 1000 ppm NH3 + 5% O2 SFG + 400 ppm NH3 SFG + 700 ppm NH3 SFG + 1000 ppm NH3 Set II Set III Set IV Set V Fig. 11. Effects of individual ﬂue gas components on Hg0 oxidation of V0.80WTiCe0.25. enhanced the catalytic activity. This indicated that promotional effect of SO2 on Hg0 oxidation was obtained with the aid of O2 (g). However, the addition of 1000 ppm SO2 into pure N2 resulted in a slight decrease of Eoxi from 47.04% to 41.85%. It was very likely that SO2 reacted with the surface oxygen to form SO3 . The reaction consumed the reactive oxygen which was active for Hg0 oxidation in the pure N2 . Thus, the inhibitive effect of SO2 would be obviously reduced by the addition of O2 (g). These behaviors were well consistent with the literature results . 3.3.4. Effect of H2O H2O inhibited Hg0 oxidation over the V0.80WTiCe0.25 due to competitive adsorption, which was consistent with the values reported in the literature . However, compared with the previous research , the reduction of Eoxi over the V0.80WTiCe0.25 was minor. This result implied that the V0.80WTiCe0.25 exhibited good resistance to H2O. 3.3.5. Effect of NH3 As a SCR catalyst, V0.80WTiCe0.25 would probably be used under SCR conditions where NH3 is usually present. Hence, it is necessary to study the effect of NH3 on the oxidation of Hg0. 1000 ppm NH3 was added to pure N2 atmosphere. NH3 concentration had a slight inﬂuence on Eoxi: the efﬁciency was 49.70% at 1000 ppm NH3 plus 5% O2 atmosphere which was slightly higher than that under 1000 ppm NH3 without O2. According to the previous study, NH3 consumed the surface oxygen that is responsible for Hg0 oxidation . Gaseous NH3 are adsorbed on the catalyst surface to form coordinated NH3 and NH2. The possible reactions over VWTiCe are proposed to be as follows: NH3ðgÞ ! NH3ðadÞ ð2Þ NH3ðadÞ þ O ! NH2ðadÞ þ OHðadÞ ð3Þ where O⁄ are active surface oxygen of the catalyst. However, many researchers have conﬁrmed that there is a competitive adsorption between NH3 and Hg0. Therefore, the competitive adsorption of NH3 and Hg0 on the VWTiCe catalyst was investigated by a desorption experiment, the results are shown in Fig. 12. V0.80WTiCe0.25 saturated by Hg0 at 250 °C under a ﬂow of Hg0 balanced in N2 was used in this test. From the ﬁgure, no obvious increase in the Hg0 concentration was observed after adding 1000 ppm NH3 and 368 L. Zhao et al. / Fuel 153 (2015) 361–369 Gaseous Hg0 was ﬁrstly adsorbed onto the active sites of catalyst to form Hg0(ad); and then the catalytic reaction is losing one oxygen atom from V2O5 to Hg0(ad) to form HgO. The consumption of V2O5 could be compensated by V2O4 bond with CeO2. Finally, the missing lattice oxygen of CeO2 would be replaced by oxygen from the ﬂue gas. The redox couples of V4+/V5+ seems to play an important role for mercury oxidation to proceed. CeO2 contains many lattice oxygen species on the surface because of the cerium in CeO2 can easily occupy two oxidation states [CeO2 (Ce4+) M Ce2O3 (Ce3+)]. Consequently, CeO2 can contribute to the redox process of V4+/V5+. It provided lattice oxygen to V2O5 to increase the valence and oxidation ability of V2O5. 4. Conclusions 0 Fig. 12. Desorption of Hg from V0.80WTiCe0.25 by NH3. turning off the Hg0 at the same time. The result demonstrates that NH3 cannot inhibit Hg0 adsorption onto the active sites. Besides, Eoxi under SCR atmosphere was also investigated. The SCR atmosphere was deﬁned as SFG plus NH3, with the NH3/NO ratio of 1. When 1000 ppm NH3 was introduced into the SFG to make SCR atmosphere, Eoxi decreased from 88.93% to 66.78%. This illustrated that the presence of NH3 inhibited Hg0 removal over the V0.80WTiCe0.25 catalyst. However, the Eoxi signiﬁcantly increased with the decrease of NH3/NO ratio. As already mentioned, NO had a promotional effect on Hg0 oxidation over V0.80WTiCe0.25 in the presence of O2. It should be noted that the SCR reaction consumed NO and O2. Thus, it would be deduced that the concentrations of NO and O2 would increase with the decrease of NH3/NO ratio as the SCR reaction occurred simultaneously with Hg0 oxidation. In another words, there would be more available surface oxygen for NO promoting the oxidation of Hg0 with the decrease of NH3/NO ratio. Nevertheless, the Eoxi of 66.78% is still encouraging, since lower space velocity and the typical ﬂue gas with HCl would result in higher Hg0 oxidation efﬁciency. 3.4. Mechanism From the results above, the catalysis mechanism for Hg0 oxidation can be explained by the Mars–Maessen mechanism. In this mechanism, adsorbed Hg0 would react with a lattice oxidant of catalyst (either O or Cl) that is replenished from the gas phase [10,56]. The active oxygen atom could produce by breaking O–O bonds on the surface of V2O5, then Hg0(ad) molecular could pick up the dissociated O atom and forming HgO. This explains why Eoxi gradually increased as O2 concentration increased. The possible mechanism for enhanced Hg0 oxidation could be explained by the following reactions: Hg0ðgÞ þ surface ! Hg0ðadÞ ð4Þ Hg0ðadÞ þ V2 O5 ! HgOðadÞ þ V2 O4 ð5Þ HgOðadÞ ! HgOðgÞ ð6Þ V2 O4 þ 2CeO2 ! V2 O5 þ Ce2 O3 ð7Þ 1 Ce2 O3 þ O2ðgÞ ! 2CeO2 2 ð8Þ The overall reactions can be summarized as follows: 1 Hg0ðgÞ þ O2ðgÞ ! HgOðgÞ 2 ð9Þ The CeO2 modiﬁed VWTi displayed excellent performance for Hg0 oxidation. Hg0 oxidation could be improved by the addition of CeO2 and V0.80WTiCe0.25 showed the best mercury oxidation efﬁciency in simulated coal-ﬁred ﬂue gas at 250 °C. NO and SO2 were observed to promote Hg0 removal with the presence of O2. The catalyst exhibited good resistance to H2O. Besides, the activity of catalysts for Hg0 oxidation decreased with the presence of NH3. Furthermore, the high activity of catalyst might ascribe to synergistic effect between V2O5 and CeO2. A likely reaction pathway for Hg0 oxidation on V0.80WTiCe0.25 was Mars–Maessen mechanism, where lattice oxygen of V2O5 reacts with adjacently absorbed Hg0. Being a novel SCR catalyst with higher Hg0 oxidation efﬁciency, further investigations will be to examine the performance of catalysts for simultaneous NOx and Hg0 removal without HCl. 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